Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer. The material layers are patterned using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's).
For many years in the semiconductor industry, optical lithography techniques such as contact printing, proximity printing, and projection printing have been used to pattern material layers of integrated circuits. Projection printing is commonly used in the semiconductor industry using wavelengths of 248 nm or 193 nm, as examples. At such wavelengths, lens projection systems and transmission lithography masks are used for patterning, wherein light is passed through the lithography mask to impinge upon a wafer.
However, as the minimum feature sizes of IC's are decreased, the semiconductor industry is exploring the use of alternatives to traditional optical lithography techniques, in order to meet the demand for decreased feature sizes in the industry. For example, short wavelength lithography techniques, Scattering with Angular Limitation in Projection Electron-beam Lithography (SCALPEL), other non-optical lithographic techniques, and immersion lithography are under development as replacements for traditional optical lithography techniques.
In immersion lithography, the gap between the last lens element in the optics system and a semiconductor wafer is filled with a liquid, such as water, to enhance system performance. The presence of the liquid enables the index of refraction in the imaging plane, and therefore the numerical aperture of the projection system, to be greater than unity. Thus, immersion lithography has the potential to extend exposure tool minimum feature sizes down to about 45 nm or less, for example.
FIG. 1 shows a perspective view of a prior art immersion lithography system or immersion exposure tool 100. An immersion exposure tool 100 such as the one shown in FIG. 1 is described in more detail in “Technology Backgrounder: Immersion Lithography,” from the website: http://www.icknowledge.com/misc_technology/Immersion %20Lithography.pdf, which is incorporated herein by reference. Immersion exposure tools are described in further detail in U.S. patent application No. 2005/0046813 A1, published on Mar. 3, 2005, for example, which is also incorporated herein by reference.
The immersion exposure tool 100 shown in FIG. 1 includes a wafer support 104 adapted to support a wafer 102. The wafer support 104 is also referred to as a wafer stage or exposure chuck, for example. A projection lens system 108 is disposed proximate the wafer 102. A fluid 106 that typically comprises de-ionized water is introduced between a last element or lens 110 of the projection lens system 108 and the wafer 102 during the exposure process, e.g., by an immersion head 120 clamped to the end of the lens system 108 or to another part of the immersion exposure tool 100. The immersion head 120 is also referred to in the art as a shower head, for example.
The wafer support 104 and the wafer 102 are moved during the patterning of the individual die or regions of die 112 on the wafer 102, e.g., from one side to another, and thus the immersion exposure tool 100 is also referred to in the art as an immersion lithography scanner. The projection lens system 108 is typically quite large and therefore usually remains stationary, for example. The wafer support 104 typically has recessed areas formed therein so that the wafer 102 is recessed when placed on the wafer support 104, as shown. One or more sensors 105 may be coupled to the wafer support 104, and may be embedded in the wafer support 104, for example, as shown. The sensors 105 are typically used for metrology, e.g., for alignment purposes, and/or illumination intensity, dose control, and laser energy measurement, although the sensors 105 may alternatively be used for other measurements. The sensors 105 may comprise an array of sensors, and may be coated with a layer of titanium nitride or other material, for example.
The fluid 106 is typically provided by a nozzle or by input and output ports within the immersion head 120, for example. During an exposure process, the fluid 106 generally continuously flows, to provide temperature stability for the immersion head and other components of the immersion exposure tool 100. In some immersion exposure tools 100, when the lens system 108 is not being used to expose the wafer 102, a closing disk 118 is used to close the end of the immersion head 120. The closing disk 118 may be disposed on the same wafer support 104 that supports the wafer 102, or the closing disk 118 may be located elsewhere in the immersion exposure tool 100 (not shown), for example.
The immersion exposure tool 100 also includes a fluid handler 114 adapted to provide the fluid 106. The fluid handler 114 may comprise a cabinet having components such as a fluid supply 116 and temperature controller, as examples, although the fluid handler 114 may also include other components, not shown. The fluid handler 114 may be coupled to the immersion head 120 by a hose 117 or other fluid-delivering means.
The wafer 102 typically includes a workpiece with a layer of radiation sensitive material such as photoresist disposed thereon. The pattern from a mask or reticle (not shown) is imaged onto the photoresist using a beam of radiation or light emitted from the lens system 108. The beam is emitted from an energy source, not shown, such as a light source, and the beam is passed through the lens system 108 to the photoresist of the wafer 102. After exposure of the photoresist, the patterned photoresist is later used as a mask while portions of a material layer (not shown) disposed over the wafer 102 are etched away (also not shown).
FIG. 2 shows a more detailed cross-sectional view of a portion 124 of the prior art immersion exposure tool 100 shown in FIG. 1 proximate the interface of the immersion head 120 and the wafer 102. The fluid 106 makes contact with a portion of the top surface of the wafer 102 and the bottom surface of the last element 110 of the projection lens system 108. The immersion head 120 includes ports 122 that may comprise an annular ring of ports for supplying the fluid 106 between the wafer 102 and the immersion head 120. The ports 122 may comprise input and output ports for injecting and removing the fluid 106, for example.
A problem with prior art immersion lithography systems 100 is that as the wafer 102 is moved beneath the last lens element 110 and immersion head 120 during the exposure process, an electrostatic charge builds up. The electrostatic charge discharges, e.g., at 126, which can damage the wafer and/or immersion head. The electrostatic charge 126 may also damage the sensors 105 or a coating on the sensors 105, for example. Such electrostatic discharge 126 causes uncertainty in the lithography process, decreases semiconductor device yields, and may require costly repairs or replacement of portions of the immersion lithography equipment, such as the sensors 105 and the immersion head 120.
Thus, what are needed in the art are methods of preventing damage from electrostatic discharge in immersion lithography systems.